This invention relates to the art of allocating available data rate to users of a wireless communication system and in particular, to allocating available data rate to users of fixed wireless loop, or so-called xe2x80x9cwireless local loopxe2x80x9d systems.
Typical prior art wireless systems employ a fixed data rate allocation per user. Once a user is assigned a modulation scheme, i.e., a constellation for mapping the user""s bits into symbols, the user""s data rate is fixed unless the user is assigned further, e.g., an additional one or more, time slots. Such systems a) maintain a fixed data rate for the user at all times provided the user is assigned no additional time slots nor loses any already assigned ones, and b) are unable to take advantage of improvements in channel quality, and suffer in the event of channel quality degradation.
We have recognized that in a system in which the modulation scheme employed may change on a per-time-slot basis, i.e., from time slot to time slot, so that the constellation used to encode the symbols of each time slot may be different for each time slot within a single frame and may be different for a particular time slot in different consecutive frames, then a user""s available data rate may change even if the user maintains the same number of time slots per frame, because the capacity of the user""s time slots may change. If one or more such changes in modulation scheme occurs, the data rate available to the user may no longer match his current data rate needs. Therefore, in accordance with the principle of the invention, we have developed a method for determining the number of time slots to assign to a user as a function of the user""s current data rate requirement and the actual current cumulative data rate of the time slots currently assigned to the user. The user data rate requirement may further be a function of a quality of service (QoS) contracted for by the user.
In particular, several modulation schemes are available, but the ability to use any particular one of them is determined by the current channel quality. Each modulation scheme provides the ability to transmit a different number of bits per symbol within a time slot, and, in accordance with an aspect of the invention, the modulation scheme employed may be changed on a per-time-slot basis. As a result, the actual available user data rate for the time slots already assigned to the user may be rapidly changing. To compensate for this, the assignment of the time slots within the frame is made dynamically and the number of time slots employed by a user is increased or decreased according to the user""s current data rate requirements and the ability of the time slots already assigned to the user to meet those requirements.